Starting April 21, we will be expanding our use of mobile-friendliness as a ranking signal. This change will affect mobile searches in all languages worldwide and will have a significant impact in our search results. Consequently, users will find it easier to get relevant, high quality search results that are optimized for their devices.
For future web development and existing web site optimization, we want to know if a site is "mobile-friendly", for which it is perhaps a good practice to check out Google's blog post entitled Finding more mobile-friendly search results.
Google provides two tools,
Google's Mobile-Friendly Web Development Guide.